What are the different types of altitude pilots need to know and when are they used?
Here's what you need to know.
This is the altitude read directly off the Altimeter in flight. Basically, it is the altitude that is being indicated.
This is the aircraft's height above Mean Sea Level or MSL. This is not the height of the aircraft above the ground below.
This is the aircraft's height above the terrain below. If there are hills or valleys, the aircraft's absolute altitude will vary accordingly.
This is the aircraft's height above the Standard Datum Plane. If the altimeter is set to the standard of 29.92, the altimeter will indicate the aircraft's pressure altitude. Pressure altitude is important as a basis for determining aircraft performance and for assigning flight levels above 18,000' above Mean Sea Level (MSL). Essentially, above 18,000' all aircraft set their altimeters to 29.92.
There are 3 ways to determine an aircraft's Pressure Altitude:
METHOD 1 - The pilot can set the aircraft's altimeter to 29.92 and read the altitude indicated.
METHOD 2 - If the pilot is not in the aircraft, he or she can use the following mathematical calculation to determine Pressure Altitude:
1- Minus the current altimeter setting from 29.92.
2 - Times that number by 1,000.
3 - Add that number to the airports field elevation.
Let's take a current altimeter setting of 30.12.
First we will subtract 30.12 from 29.92 and get -0.20.
We will then take -0.20 and multiply it by 1,000 to get -200'.
We will then subtract 200' from the airport field elevation.
If the airport field elevation was 4,473' the pressure altitude would be 4,273'.
METHOD 3 - The pilot can use the following chart from PHAK Chapter 11 to determine Pressure Altitude.
Density Altitude is used in calculating aircraft performance data. It is essentially the altitude the aircraft will be performing at, regardless of its actual altitude. For example, an aircraft may be flying at an altitude of 5,000' MSL. However, if the density altitude that day is 7,000' then the aircraft will be performing as if it were at 7,000' not 5,000'.
Think of it like this... the higher the density altitude, the worse the aircraft will perform. The lower the density altitude, the better the aircraft will perform.
There are multiple weather factors that will effect the outside density of the air and therefore effect the density altitude at which the aircraft is flying.
1 - Hotter Temperatures will increase density altitude while cooler temperatures decrease density altitude.
2 - Increased water vapor in the air will increase density altitude, while drier/less humid air will decrease density altitude.
3 - Outside air pressure will also effect density altitude. The higher the air pressure, the more dense the air, the lower the density altitude. Conversely, lower pressure leads to less dense air and a higher pressure altitude.
Remember, high density altitude is bad for aircraft performance!
Pilots can calculate their density altitude by utilizing the chart below:
Author - Nate Hodell
CFI/CFII/MEI/ATP - Creator of wifiCFI - Owner of Axiom Aviation Flight School.
This information is included in the Performance and Limitations Lessons on wifiCFI. Sign up today to watch videos, listen to podcasts, take lesson quizzes, join live webinars, print lesson quicktakes, and more by clicking this link >